FIG. 1 shows a block diagram of a typical ultrasound imaging system. It operates as follows: under the control of a main controller, a probe transmits an ultrasound to a body tissue to be tested, and receives an echo signal reflected from the body tissue after a certain delay. The echo signal is fed into a beam former, which performs focus for delay, weighting and channel accumulation to generate signals on one or more scan lines. A detector detects the scanning signals output from the beam former and feeds them into a DSC (Digital Scanning Converter), where a coordinate conversion is implemented. The resulted image data is sent to a computing device (generally including a built-in processor, a FPGA circuit, even a computer system or the like). An image enhancement module located in the computing device processes the image data and feeds it into a monitor for display. Alternatively, the ultrasound imaging system may invoke an image sequence stored in an external memory, process them by means of the image enhancement module and then display them.
Here, the ultrasound imaging system employs the image enhancement module to realize post-processing of image, which aims at improving the quality of ultrasound images and assisting medical diagnosis, in particular, by overcoming problems in two aspects. The first aspect is to enhance significant structures or features in the image which interest the doctors, including bones, capsules, canals, cavities or the like. That is to say, all distinguishable structures should remain in the resulted image after ultrasound image enhancement, including normal and abnormal structures, while providing sufficient textural and contrast information. In the second aspect, speckles should be suppressed. If the reflection surface of a tissue within the human body is not so smooth that the coarseness of surfaces are equal to the wavelength of the incoming ultrasound, the echo signals generated by different reflection sources may overlay or counteract due to phase difference. Such an overlay or counteraction is represented visually as grains of the image. As such, speckle noises are always present when scan line data at different locations are processed and combined to form a final ultrasound image. The speckle noises will mask some useful information in the image, thus interfering with the doctor's diagnosis to some extent. Speckle mitigation is thus another object for enhancing image.
In relation to the above problems, an image enhancement method with a gradient-based segmentation is disclosed in U.S. Pat. Nos. 6,208,763 and 6,592,523. In this method, an image is segmented into a structural and a non-structural region according to the gradient information. Anisotropy sharpening is performed on the structural region based on Intensity-weighted 2nd order directional derivative, to enhance contrast of edges of the image. Isotropic smoothing is performed on the image data classified as a non-structural region, to mitigate speckle noise. The above prior art, however, has the following disadvantages: (1) although the method in the patent advantageously segments an image into a structural and a non-structural region, but it leaves out some inconspicuous features by only taking gradient scale information into account, in particular, the enhancement effect relies too much on the segmentation template when the structural region takes the gray-scale of the image as the sharpening coefficient, and too much discontinuity will occur at the edges of the segmentation template. Accordingly the enhancement effect is not ideal; and (2) linear smoothing of the non-structural region cannot mitigate speckle noises greatly.